Robinson Annulation

The
Robinson Annulation is a useful reaction for the formation of
six-membered rings in polycyclic compounds, such as steroids. It
combines two reactions: the
Michael Addition and the Aldol Condensation

Mechanism of the Robinson Annulation

The first step in the process is the Michael Addition to an α,β-unsaturated
ketone, such as methyl vinyl ketone (ethyl vinyl ketone is shown above):

The newly formed enolate intermediate must first tautomerize for the
conversion to continue:

The Robinson Annulation can also proceed under acidic catalysis, with the
entire process occurring in one pot, as shown below. The use of a precursor of
the α,β-unsaturated ketone, such as a β-chloroketone, can reduce the
steady-state concentration of enone and decrease the side reaction of
polymerization.